A vehicle air-conditioning system mainly includes a compressor, a condenser, a throttling element, and an evaporator. A gaseous refrigerant with high temperature and high pressure discharged from the compressor changes into a liquid refrigerant after being condensed by the condenser. The liquid refrigerant is throttled and depressurized by the throttling element, and then enters into the evaporator to exchange heat, in the evaporator, with air outside the evaporator, and changes into the gaseous refrigerant and then flows back into the compressor, thus accomplishing a refrigeration cycle. The air enters into a vehicle compartment after being cooled by the evaporator, and if the temperature in the vehicle compartment is lower or higher than a preset temperature in the vehicle compartment, a superheat degree is controlled by controlling and regulating an opening degree of the throttling element opening degree.
In a traditional engine automobile, a compressor of an air-conditioning system is directly connected to an engine via a belt, thus a rotational speed of the compressor cannot be controlled. With the gradual popularization of energy-saving and environmental-friendly hybrid automobiles and electric automobiles, more and more automobiles adopt an electric compressor to replace a traditional pulley driven compressor to drive the compressor for providing a motive force for the air-conditioning refrigeration cycle. And the conventional automobile air-conditioning systems mainly employ a thermal expansion valve as a throttling element, and when the thermal expansion valve is used in the electric compressor system, the following problems exist.
An action of a valve needle of the thermal expansion valve is driven by a pressure difference between a saturated pressure corresponding to a bulb in the valve and a pressure in the pipeline, and is completely decided by a mechanical force. Therefore, the thermal expansion valve cannot respond rapidly and correctly to a rapid change of the working condition of the compressor or the evaporator fan. The thermal expansion valve cannot determine the corresponding opening degree according to various parameters of the system, and controlling of the superheat degree is not stable, thus adversely affecting a comfort degree of an air-out temperature. Most of the current new energy automobiles require the air-conditioning system to be not only applied to cool a compartment, but also to cool a battery and a frequency converter, and in the case that the thermal expansion valve is employed in such a double-evaporator type air-conditioning system, the thermal expansion valve cannot be fully shut off during operation of the system, thus fluid still flows in another side of the air-conditioning system when only the evaporator or the heat exchanger works, thereby adversely affecting the efficiency. Also a large amount of liquid is accumulated in low-pressure pipelines at a non-working side, resulting in increasing of a charge amount of the system, thus the system requires to use a relative large liquid accumulator, which increases the cost.
Replacing the thermal expansion valve as a throttling element in the air-conditioning system with the electronic expansion valve has the following advantages.
The electronic expansion valve may be regulated according to various parameters in the air-conditioning system, and the corresponding control strategy may be timely adjusted according to various working conditions, to achieve the objects of improving the efficiency of the cooling system, and saving energy and protecting environment. Controlling the superheat degree by the electronic expansion valve is smoother than controlling the superheat degree by the thermal expansion valve, thus allowing the air-out temperature to be stable, and the comfort degree is improved.
The current household or commercial air conditioners adopt the electronic expansion valve as the throttling element, since the working conditions of the household or commercial air conditioners are stable and the load of a cooling system changes a little, the household or commercial air conditions generally simply employ the superheat degree as a proportion integration differentiation (PID) input parameter to control the opening degree of the electronic expansion valve, to control a refrigerating capacity. However, the automobile air-conditioners are different from the household or commercial air-conditioners, and the working condition of the vehicle air conditioners change rapidly, thus adjusting the electronic expansion valve simply by using the superheat degree may have problems of a slow response speed, and being easily overregulated.
In disclosed patents (such as the patent No. 200510021304.7), a control method for an air-conditioning system of an electric automobile using an electronic expansion valve as the throttling element is put forward, a rotational speed of the compressor and the opening degree of the electronic expansion valve are adjusted by a temperature sensor in the compartment in the control method, however, this control method for the air-conditioning system does not take parameters such as the speed of the evaporator fan and the temperature and pressure of the system into consideration, and in the case that the working condition changes, various issues such as degrading of system efficiency, liquid hammer of the compressor may be caused.